ESTRO 36 Abstract Book

567 / 1082

S552 ESTRO 36 2017
_______________________________________________________________________________________________
rotations, intrafraction variation, delineation, treatment
and match uncertainties.
When applying online position verification on the prostate,
setup margins for lymph nodes must be 0.19 cm (lateral),
0.73 cm (longitudinal) and 0.57 cm (vertical). When
applying online setup correction on bony structures, setup
margins for the prostate must be 0.25 cm, 0.73 cm and
0.73 cm respectively. Offline setup correction on prostate
resulted in the largest margins for both volumes.
Conclusion
The required setup margin depends on the applied
correction strategy. When applying position verification
on bony structures, larger margins are required for CTV
prostate. When applying position verification on prostate,
larger margins are required for CTV lymph nodes. When
applying these margins clinically, additional margins are
needed to account for rotations, intrafraction variation,
delineation, treatment and match uncertainties.
PO-1010 Investigation of reproducibility of bolus
position based on kV CBCT imaging
E. Dabrowska1,2, P. Czuchraniuk 3, M. Gruda1, P.
Kukolowicz1, A. Zawadzka1
1 Maria Sklodowska-Curie Memorial Cancer Center and
Institute of Oncology, Department of Medical Physics,
Warsaw, Poland
2 Faculty of Physics University of Warsaw, Department of
Biomedical Physics, Warsaw, Poland
3 Maria Sklodowska-Curie Memorial Cancer Center and
Institute of Oncology, Department of Radiotherapy,
Warsaw, Poland
Purpose or Objective
On-board kilovoltage (kV) Cone Beam Computed
Tomography (CBCT) imaging is being used predominantly
for patients' positioning to improve the precision and
accuracy of treatment delivery. Moreover, volumetric
CBCT images can be used to evaluate anatomy changes
like tumour shrinking or weight loss as well as changes in
organ volume, deformation or position. Also
reproducibility of accessories used in radiotherapy (the
vacuum bag, bolus position, etc.) can be monitored.
Additionally, the CBCT data set can be used for calculation
of dose distribution. The aim of our study was to analyze
reproducibility of bolus’s position based on kV CBCT
imaging and compare planned and delivered dose
distribution in Clinical Target Volume (CTV).
Material and Methods
For 10 post-mastectomy patients, 35 sarcoma patients and
5 patients with vulva cancer the treatment CT based plans
with bolus were prepared (Eclipse, Varian). For the post-
mastectomy patients the planning CT was acquired with
bolus. For the two other groups the planning CT was made
without bolus. Bolus was drawn in the treatment planning
system. For each patient CBCTs were acquired in the first
and mid fraction and also at the end of treatment. CBCTs
were co-registered offline (automatic rigid match) to the
planning CT. The correctness of boluses positions were
evaluated. Also dose distributions were calculated with
CBCT images and compared with dose distributions
obtained with planning CT. For each patient we took a
photo to document the bolus’s position. To compare dose
distribution calculated on CT and CBCT, a new HU-
density calibration curve was measured and introduced
into treatment planning system. CatPhan®503 phantom
was used.
Results
Fusions of CTs and CBCTs showed that there are several
different problems with reproducibility of bolus position.
First of all, bolus generated in TPS will never adhere to
skin like it is presented in TPS, especially when a
irregularity of patient surface is high (Fig. 1a). Moreover,
usually air gaps occur even when there is a smooth surface
of the patients’ body (Fig. 1b). Another problem is the
compatibility of bolus position relative to a field edge (Fig.
1c) which is difficult to reproduce despite it is accurately
described in patient folder. The last two discrepancies
appear regardless if bolus was placed on patient’s skin
during CT scanning or generated in TPS. Preliminary
calculations for 10 soft tissue patients treated with 3D-
CRT plan show, that there is 5,9% ±2,0% a discrepancy
between D98% calculated on CT and CBCT. More advanced
data analysis will be presented for each location
separately, for treatment planning techniques and
information about taken/not taken photo before CT
scanning.
Conclusion
CBCT is a very useful method for accuracy of treatment
planning verification. It allows not only patient position
verification but also for evaluation of bolus positioning
accuracy. Based on CBCT several mistakes influencing
dose deposited to CTV were revealed, what was not
noticeable during a routine verification based on two-
dimensional orthogonal images.
PO-1011 Is it safe to omit a setup correction
validation scan for central lung lesions treated with
SBRT?
M.M.G. Rossi1, H.M.U. Peulen1, J.S.A. Belderbos1, J.J.
Sonke1